1. Field of the Invention
The present invention relates to a method for connecting a mobile station to a base station in a mobile communication system including both base stations capable of directional beam signal transmission and reception and base stations incapable of directional beam signal transmission and reception, and a radio network controller and a mobile station utilizing the method.
2. Description of the Related Art
In a communication system utilizing DS-CDMA (Direct Sequence-Code Division Multiple Access), multiple mobile stations work in the same frequency band and communicate with a base station. In the DS-CDMA communication system, signals transmitted from or received by the mobile stations are identified by spread codes. For example, Gold codes or other orthogonal codes may be used as the spread codes.
Considering a case in which a specific mobile station transmits signals to or receives signals from a base station, in the course of spectrum despreading, signals transmitted between the base station and other mobile stations act as interference signals with respect to the signals transmitted between the specific mobile station and the base station (below, the latter is referred to as target signal). The electric power of the interference signals is on average equal to one over a processing gain (PG).
In a non-synchronized uplink communication environment in which signals are transmitted from mobile stations to a base station, the signals transmitted from the mobile stations are subject to instantaneous fluctuations, short interval fluctuations, and distance fluctuations caused by fading. Therefore, in order that the communications between the specific mobile station and the base station possess required quality, transmission power of the specific mobile station should be adjusted so that the ratio of strength of signals from the specific mobile station to strength of signals from the other mobile stations is constant. This ratio is so-called SIR (Signal to Interference power Ratio).
However, even if the transmission power of the specific mobile station is controlled perfectly and the SIR of signals received by the base station is maintained to be constant, in a multi-path communication environment, the spread codes are not completely orthogonal codes. As a result, the target signals from the specific mobile station are subject to interference caused by electric power cross-correlation of other mobile stations, and the strength of the interference due to each of the other mobile stations is equal to one over the processing gain (PG) on average. Accordingly, if the number of mobile stations working in the same frequency band increases, the level of the power of the interference signals increases. Consequently, in the related art, the number of mobile stations operable in each cell is limited by the reception characteristics determined by the required communication quality.
In order to increase the number of mobile stations operable in each cell, interference cancellation techniques are employed, which involve reduction of the cross-correlation. One example of these techniques is the so-called “adaptive antenna array diversity” technique. In this technique, multiple antennas are used to transmit and receive signals. To the signals received by each of the antennas, an appropriate weighting factor is applied, and the resultant signals are synthesized. This enables reduction of the interference of signals from other mobile stations with the target signals from the specific mobile station. For example, a reception method based on adaptive antenna array diversity is discussed in “Pilot symbol-assisted decision-directed coherent adaptive array diversity for DS-CDMA mobile radio reverse link”, by S. Tanaka, M. Sawahashi, and F. Adachi, IEICE Trans. Fundamentals, Vol. E80-A, pp. 2445–2454, December 1997.
FIG. 1 is a view showing an example of a configuration of a base station-employing the adaptive array diversity reception method.
The base station shown in FIG. 1 includes a number of m antennas 200-1 through 200-M, a RF radio set 202, a number of m matched filters 204-1 through 204-M, a weighting factor controller 205, a number of m multipliers 206-1 through 206-M, an accumulator 208, a phase fluctuation estimation unit 210, a multiplier 212 for compensating the phase fluctuation, an identification determination unit 214, an accumulator 216, a multiplier 218 for estimating the phase fluctuation, and a measurement unit 220 for measuring the ratio of the interference signal power to the target signal power.
In the base station shown in FIG. 1, signals received by the antennas 200-1 through 200-M are detected in the RF radio set 202 by quadrature detection. The matched filters 204-1 through 204-M are in conjunction with the antennas 200-1 through 200-M, and de-spread the output signals from the RF radio set 202, therefore estimating the reception code sequence.
The weighting factor controller 205 calculates the weighting factors to be applied to the output signals from the matched filters 204-1 through 204-M, respectively, based on the output signals from the matched filters 204-1 through 204-M and the output signals from the multiplier 218 for estimating the phase fluctuation.
The multipliers 206-1 through 206-M apply the calculated weighting factors to the output signals from the matched filters 204-1 through 204-M, respectively. The accumulator 208 synthesizes the output signals from the multipliers 206-1 through 206-M and outputs the synthesized signal.
The multiplier 212, which is for compensating the phase fluctuation, multiplies the output signal from the phase fluctuation estimation unit 210 with the output signal from the accumulator 208 so as to perform phase compensation. The identification determination unit 214 receives the output signals from the multiplier 212, and outputs the final received data (reproduced data). The accumulator 216 outputs the difference between the input and output signals. The multiplier 218, which is for estimating the phase fluctuation, multiplies the output signals from the phase fluctuation estimation unit 210 and the output signals from the accumulator 216, and outputs the resultant signals to the weighting factor controller 205.
The measurement unit 220 measures the ratio of the power of the target signals to the power of the interference signals (SIR) based on the output signals from the accumulator 208. Further, the measurement unit 220 compares the measured SIR with a predetermined reference SIR, and generates a control signal for adjusting the transmission power of the mobile station concerned to make the actual SIR equal to the reference SIR.
The base station transmits common pilot channel signals using an omnidirectinal beam to all the mobile stations in a cell. The common pilot channel signals are used for channel estimation and measurement of the reception power for all mobile stations in the cell. Meanwhile, the base station uses dedicated pilot channels to send specific information to each mobile station in the cell. The base station transmits the dedicate pilot channel signals using a directional beam or an omnidirectinal beam.
FIG. 2 illustrates an example in which an omnidirectional beam 250 is used for transmitting both the common pilot channel signals and the dedicated pilot channel signals. FIG. 3 illustrates an example in which a directional beam 251 is used for transmitting the dedicated pilot channel signals, and an omnidirectional beam 250 is used for transmitting the common pilot channel signals.
In the base station shown in FIG. 1, which employs the adaptive array diversity reception method, signals received by multiple antennas are de-spread, then the de-spread signals from each antenna are multiplied by an antenna weighting factor, and then are synthesized. Further, the base station makes adjustments so that the SIR of the synthesized signals becomes the maximum. The dedicated pilot channel signals are transmitted by an omnidirectional beam, and this improves the SIR of the received signals at the base station and all mobile stations, and results in better quality of the received signals.
In a cellular mobile communication system including a number of base stations, each base station forms a cell to cover a service area of the cellular mobile communication system. Further, in a cellular mobile communication system employing DS-CDMA, communications are performed using different spread codes, enabling communication with the same carrier wave frequencies in all cells.
In a cellular mobile communication system, usually a mobile station is positioned in a cell, and via a radio link communicates with the base station that forms the cell. When the mobile station moves in a region overlapped by a number of cells, the mobile station communicates with the base stations that form these cells via the radio link. When the mobile station further moves out of the overlapping region and into a region of a cell formed by a single base station, the mobile station communicates with the single base station via the radio link. This procedure is called “handover”.
FIG. 4 is a view showing the relation between the position of a mobile station (MS) and the reception power of the common pilot channel (CPICH) signals in the course of handover. The reception power of the common pilot channel (CPICH) signals means the power of the common pilot channel (CPICH) signals received at the mobile station.
The graph in FIG. 4 is explained below.
At certain time intervals, the mobile station measures the reception power of the common pilot channel signals from a number of base stations in the surrounding area transmitted at certain transmission power levels, and reports the results to a RNC (radio network controller) for controlling the radio link connection between the mobile station and the base stations. According to the measured reception power of the common pilot channel signals from the base stations in the surrounding area, the mobile station selects a base station yielding the largest reception power of the common pilot channel signals, connects to the base station via the radio link, specifies the setting of the dedicated pilot channel and transmits information to or receives information from the base station.
As shown in FIG. 4, when the mobile station is within the service area of a base station BS1 but out of the service area of a base station BS2, the reception power of the common pilot channel signals transmitted from the base station BS1 is larger than the reception power of the common pilot channel signals transmitted from the base station BS2. Therefore, the mobile station connects to the base station BS1 through the radio link, specifies the setting of the dedicated pilot channel and transmits information to or receives information from the base station BS1.
Then, when the mobile station moves away from the base station BS1 and close to the base station BS2, the reception power of the common pilot channel signals transmitted from the base station BS1 decreases, and the reception power of the common pilot channel signals transmitted from the base station BS2 increases, and the difference between the reception power of the common pilot channel signals transmitted from the base station BS1 and the reception power of the common pilot channel signals transmitted from the base station BS2 decreases gradually. When the difference becomes smaller than a predetermined threshold value (handover threshold, or specifically handover addition threshold), the RNC specifies the base station BS2 as an additional radio link connection destination of the mobile station. Therefore, the mobile station connects to the base station BS2 through the radio link, specifies the setting of the dedicated pilot channel and transmits information to or receives information from the base station BS2. Hence, the mobile station now communicates with both the base station BS1 and the base station BS2.
FIG. 5 is a schematic view showing the control procedure when a mobile station is communicating with multiple base stations. In uplink communications, base stations 302-1 and 302-2 each receive signals from a mobile station 300, and demodulate the signals. Further, the base stations 302-1 and 302-2 transmit the demodulated signals together with reliability information to the higher-ranking RNC 306 through the uplink cable transmission channel 308-2 and 308-1, respectively. The RNC 306 selects the demodulated signals from the base stations 302-1 and 302-2 and synthesizes them based on reliability information. Consequently, the quality of the received signals in the uplink communications is improved.
On the other hand, in downlink communications, the RNC 306 transmit the same signals to the base stations 302-1 and 302-2 through the downlink cable transmission channel 308-4 and 308-3, respectively, and the base stations 302-1 and 302-2 receive the signals at the same time. The mobile station 300 receives signals from the base stations 302-1 and 302-2, and synthesizes the signals. Consequently, the quality of the received signals in the downlink communications is improved.
Returning to FIG. 4 to continue the explanation, when the mobile station, which is communicating with both the base station BS1 and the base station BS2, moves further away from the base station BS1 and close to the base station BS2, the reception power of the common pilot channel signals transmitted from the base station BS2 becomes larger than the reception power of the common pilot channel signals transmitted from the base station BS1, and the difference between the reception power of the common pilot channel signals transmitted from the base station BS2 and the reception power of the common pilot channel signals transmitted from the base station BS1 increases gradually. When the difference reaches a predetermined threshold (handover threshold, or specifically handover deletion threshold), the RNC deletes the base station BS1 from the list of the radio link connection destinations of the mobile station. According to the instruction of the RNC, the base station BS1 disconnects the radio link with the mobile station. Then the mobile station communicates with the base station BS2 only.
Turning to another issue of the conventional cellular mobile communication system, when a mobile station is on standby, the mobile station selects one base station and connects to the base station via the radio link. When the mobile station moves, along with the movement, the mobile station disconnects the present radio link connection with the selected base station, and connects to a next base station via the radio link, and repeats the base station switching operation sequentially in the same way.
FIG. 6 is a view showing the relation between the position of a mobile station (MS) and the reception power of the common pilot channel (CPICH) signals when the mobile station (MS) is on standby. Here, it is assumed that the transmission power levels of the base stations BS1 and BS2 are the same.
At certain time intervals, the mobile station measures the reception power of the common pilot channel signals from a number of base stations in the surrounding area transmitted at certain transmission power levels, selects a base station yielding the largest reception power of the common pilot channel signals and connects to the base station via the radio link. Then the mobile station is on standby, that is, continues to receive the common pilot channel signals and waits to receive communication signals from the base station.
As shown in FIG. 6, when the mobile station is within the service area of the base station BS1 but out of the service area of a base station BS2, the reception power of the common pilot channel signals transmitted from the base station BS1 is larger than the reception power of the common pilot channel signals transmitted from the base station BS2. Therefore, the mobile station connects to the base station BS1 through the radio link and is on standby.
Then, when the mobile station moves away from the base station BS1 and close to the base station BS2, the reception power of the common pilot channel signals transmitted from the base station BS1 decreases, and the reception power of the common pilot channel signals transmitted from the base station BS2 increases, and the difference between the reception power of the common pilot channel signals transmitted from the base station BS2 and the reception power of the common pilot channel signals transmitted from the base station BS1 increases gradually. When the difference becomes smaller than a predetermined value (cell selection threshold), the mobile station disconnects the radio link with the base station BS1 and connects to the base station BS2, being on standby.
Meanwhile, when the difference between the reception power of the common pilot channel signals transmitted from the base station BS1 and the reception power of the common pilot channel signals transmitted from the base station BS2 becomes larger than the cell selection threshold, the mobile station disconnects the radio link with the base station BS2, connects to the base station BS1, and continues the standby state.
Turning to the problem to be solved by the present invention, as described above, in order to improve the quality of the received signals at a base station and a mobile station, it is preferable that the mobile station preferentially connect the radio link with a base station capable of directional signal transmission and reception. However, in the related art, as described above, when switching base stations in a handover process or when the mobile station is on standby, it is not taken into consideration whether the base station is capable of directional signal transmission and reception, hence an appropriate control is not performed to further improve the quality of the received signals at the base station and the mobile station.